A composite structure, as used herein, may be any structure comprised of at least one composite material. The composite material may include a matrix material and a reinforcement material. Composite structures may be inspected using different types of testing systems. A laser ultrasound testing system is an example of one type of testing system that may be used to non-destructively evaluate or inspect a composite structure for undesired inconsistencies. Laser ultrasound testing allows a composite structure to be tested without the testing system physically contacting the composite structure.
Laser ultrasound testing is a rapid, non-contact, non-couplant alternative to traditional ultrasound testing. Laser ultrasound testing may be used to inspect composite structures having complex contours such as those structures found in, for example, the aerospace industry. High power laser ultrasound testing systems have shown distinct benefits over conventional ultrasound testing but are very expensive and have a large footprint which requires significant floor space. Low power, high repetition rate, fiber-based ultrasound testing systems are being developed and have a distinct advantage of costing less than high power laser ultrasound testing systems. However, the low power laser ultrasound testing systems have limits as to how much energy can be input to the composite or other structures being tested, limiting the signal to noise ratio and depth of penetration.
Existing solutions for increasing the signal to noise ratio with laser ultrasound testing is averaging. For example, the signal to noise ratio of a laser ultrasound testing system can be improved, so that smaller inconsistencies can be found, by averaging multiple laser pulses at an inspection point. However, averaging the multiple laser pulses considerably slows down the scanning process. Further, overuse of averaging may generate excessive heat within the composite structure. For example, sustained laser pulses at one location increases the temperature of the composite structure at the laser impingement point. Improving the power density by focusing the laser spot can generate a strong stress pulse and improve the depth of penetration of incident energy within the composite structure however, focusing the laser spot may also generate excessive heat within the composite structure.